Vibrational spectrum perturbations of alkanethiol self-assembled monolayers with noble gases and chlorinated species

Abstract

This study aims to improve our understanding of some differences in odd and even chain-length alkanethiol self-assembled monolayers (SAMs) using infrared reflection absorption spectroscopy (IRRAS) and density functional theory (DFT). Xe, Kr, CH2Cl2, CD2Cl2, and CDCl3 were used experimentally to perturb the CH2 and CH3 stretches arising from the tail region of the SAM films and changes in position and intensity were monitored in the infrared. Using DFT methods, noble gases and CH2Cl2 were added to models of the SAM monolayers, and energies and vibrational spectra were calculated. It was observed that perturbing species affected the CH3 symmetric and asymmetric stretches on hexadecanethiol SAMs (an “even” SAM with a 16-carbon backbone) while on pentadecanethiol SAMs (an “odd” SAM with a 15-carbon backbone) both CH2 and CH3 symmetric and asymmetric stretches were affected. Films formed with shorter chain-length species, hexanethiol (even) and pentanethiol (odd), had less consistent results, likely due to more disorder in the alkanethiol chains from weaker van der Waals interactions. Adsorption energies for different perturbing species on the monolayers were higher for the hexanethiol than the pentanethiol. The vibrational spectrum of pentanethiol monolayers with adsorbed species mainly showed shifts to higher frequencies for CH2 and CH3 stretches; for hexanethiol, shifts to lower frequencies for CH2 stretches and higher frequencies for CH3 stretches were observed. Thus, the odd and even alkanethiol SAMs were affected differently by the perturbing species as observed both experimentally and computationally.